Need Help With Experiment

David R. Boone boone at ese.ogi.edu
Wed Apr 17 09:41:14 EST 1996

Terry J. Peek wrote:
> I am trying to differentiate between Serratia marcescens and a gram pos. cocci
> that I suspect is Staph epidermidis or S. aureus. I've placed the culture on
> 10% NaCl agar. I'll check the results later this morning. What is the best way
> to seperate these organisms using the resources of a small college lab? Is my
> first step in the right direction? Any suggestions would be appreciated.
> Thanks.       Terry

There are lots of differences between Serratia marcescens and the two cocci you 
mentioned.  If you know an organism is one of these three species, it is 
relatively easy to distinguish Serratia marcescens.  Many strains of Serratia 
marcescens when grown as surface colonies produce a red pigment (in contrast, 
Staphylococcus epidermidis is usually light colored and Staphylococcus aureus is 
usually dark yellow or golden (the species epithet refers to gold).  The color of 
colonies is really not very taxonomically significant (in fact, <90% of Serratia 
marcescens strains produce the red pigment), but if you want a quick and dirty 
discriminator, this may be it.  From a taxonomic standpoint, probably the most 
reliable differentiating characteristics are morphological: Serratia are 
gram-negative rods, whereas staphylococci are gram-positive cocci, often arranged 
in grape-like clusters.

David R. Boone
Professor of Environmental Microbiology
Oregon Graduate Institute, Portland
boone at ese.ogi.edu

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